This invention relates to a method of applying slip for an enamel coating to interior surfaces of a tank such as a heat exchanger of a hot water boiler, electric water heater, hot water storage tank, etc., and more particularly to a method of applying the slip to the interior surfaces of a tank after it is fabricated as by welding. Specifically, the invention is directed to the provision of a heat exchanger of a hot water boiler with a hot water resisting enamel coating.
In this specification, the term "closed tank" refers to such a tank that has openings, such as nipples, of which areas are small as compared with the internal volume of the tank, and a closed tank is hereinafter referred to as a tank. Also, porcelain enameling is a general term for both porcelain enameling and glass lining.
In the prior art porcelain enameling processes, slip is applied to various components of the tank by spraying before they are assembled into a tank, and the components are baked and assembled together by welding to provide an enameled tank.
According to the prior art processes, it is possible to provide an enamel coating of a desired film thickness, and defects in an enamel coating can be readily detected because enamel application is separately effected for each component of the tank. Thus the component with defects in enamel coating can be subjected to enamel application again. Therefore, these processes are now being used widely, but are disadvantageous in that manufacturing costs are high, as described hereinbelow. Since slip is applied by spraying, it scatters and its rate of recovery is low. Each part of the tank must be baked in a furnace. The parts of the tank must be relieved of enamel at such portions thereof to be welded. In addition, these processes have the following disadvantages. The portions of the parts which have been relieved of the enamel coating may become defective due to the fact that they tend to be brought into contact with the liquid in the tank because of misalignment which would arise when the parts are joined. A weld bead (backing bead) at the joints tends to be exposed. Also, when a tank is fabricated, one component is force fitted into another component and then is welded thereto. There is a danger of the enamel coating chipping off during force fitting.
As described above, the conventional method for enamel application requires that after slip application the various components of a tank be subject to force fitting and welding. Therefore, various problems have been encountered in carrying out this method.
In order to overcome these disadvantages, it has been proposed to perform slip application after a tank is fabricated, thereby eliminating force fitting and welding operations after porcelain enameling. When such measures are employed, it is essential that slip application be completely performed since it is impossible to inspect porcelain enameling over the interior surfaces of the closed tank from outside.
A proposal has been made to first fabricate a tank and then to apply slip by shooting wet slip through hoses into the tank so that `flow coating` can be effected, as described in `NEW LINE PORCELAIN ENAMELS WATER HEATERS` of `THE INTERNATIONAL ENAMELIST`, 17[4] pages 3 to 6, published October 1967. This method requires skills in carrying it into practice, and is not suitable for a tank having an inner shell and an outer shell, such as the tank of a hot water boiler.
It is considered likely that if slip of a smaller amount than the internal volume of a tank is introduced into the tank, the tank is rotated for effecting slip application and then excess slip that has not been used for slip application is drained from the tank, it will be possible to provide an enamel coating of a desired uniform thickness. However, the results of experiments show that it is impossible to provide an enamel coating of a desired uniform thickness by this method. Moreover, two problems are encountered in carrying out this method. One is that some surfaces of the tank are not substantially entirely coated with enamel, and the other is that the slip applied to the surfaces to form an enamel coating of a desired thickness tends to slump or sag, with the result that no enamel coating of a uniform thickness can be formed. In the aforementioned type of tank, it is generally believed that an enamel coating must have a thickness of over 120.mu..